1 Some ions are made up of several non-metallic atoms joined together. These are
called polyatomic ions ("poly" means "many"). Consider the hydroxide ion. In the
compound NaOH, for example, the sodium has a charge of 1+. The oxygen and
hydrogen together form the polyatomic hydroxide ion, OH, with a charge of 1-. Human
bones contains calcium phosphate. The phosphate anion, PO , behaves like a single
ion with a charge of 3-.
2 Suffixes for Polyatomic Ions
2.1 The two most common suffixes used in naming polyatomic ions are "-ate" and
"-ite." When a pair of similar ions exist, such as SO and SO , "-ate" and "-ite"
are used in their names to differentiate them. SO is sulfate, and SO is sulfite.
As you may have guessed, "-ate" means more oxygen atoms, and "-ite"
means fewer oxygen atoms are part of the ion. But these suffixes do not tell
you how many oxygen atoms are actually in the formula. If there are more
than two similar ions, then other naming variations are used. It is not
necessary at this stage to memorize all the suffiix patterns.
3 Naming Compounds Containing Polyatomic Ions
3.1 Naming a compound containing polyatomic ions is simple. Look at the formula,
and name the cation, followed by the anion. You do not need to change the
ending of a polyatomic ions name. The only difficulty sometimes is recognizing
the ions within the formula. The polyatomic ion is often found inside brackets.
4 Writing Formulas for Compounds Containing Polyatomic Ions
4.1 The rules for writing the formulas for these compounds are similar to the rules for other
ionic compounds. One difference is that brackets may be used to show the ratio of the
ions. The subscript outside the brackets applies to all the elements inside the brackets. For
example, in Fe (SO ) the subscript 3 with (SO ) means that there are 3 SO ions for every 2
Fe ions in the compound. It also indicates that 3 sulfur atoms and 12 (4 x 3) oxygen atoms
are in one formula unit of sulfate.
4.1.1 Example Problem: What is the formula for iron(III) sulfate?
188.8.131.52 1. Identify the ions and their charges.
184.108.40.206.1 Iron(III): Fe
220.127.116.11.2 Sulfate: SO
18.104.22.168.3 2. Determine the total charges needed to balance.
22.214.171.124.3.1 Fe : 3 + 3 = 6
126.96.36.199.3.2 SO : 2 + 2 + 2 = 6
188.8.131.52.3.3 3. Note the ratio of cations to anions.
184.108.40.206.3.3.1 2 to 3
220.127.116.11.3.3.2 4. Use brackets and subscripts to write the formula .
18.104.22.168.22.214.171.124 Fe (SO )
4.1.2 Example Problem: What is the formula of ammonium dihydrogenphosphate?
126.96.36.199 1. Identify the ions and their charges.
188.8.131.52.1 Ammonium: NH
184.108.40.206.2 Dihydrogenphosphate: H PO
220.127.116.11.3 2. Determine the total charges needed to balance.
18.104.22.168.3.1 NH : 1
22.214.171.124.3.2 H PO : 1
126.96.36.199.3.3 3. Note the ratio of cations of anions.
188.8.131.52.3.3.1 1 to 1
184.108.40.206.3.3.2 4. Write the formula. Brackets and a subscript are not needed when there is only one ion.
220.127.116.11.18.104.22.168 NH H PO
22.214.171.124.126.96.36.199 The formula of ammonium dihydrogenphosphate is NH H PO .